Heme serves Salmonella typhimurium as the cofactor both for cytochromes (required for respiration) and for catalase (involved in defense against oxygen's toxic effects). In S. typhimurium, the heme pathway also leads to siroheme and to vitamin B 12, which is an essential nutrient for humans. Surprisingly, very little is known about the regulation of this branched pathway in enteric bacteria. Since S. typhimurium well-suited to genetic analysis, we propose a primarily genetic approach to understanding the control of heme synthesis. We have been investigating the role of the hemA and hemL genes in the synthesis of ALA, the first committed, stable intermediate in the heme pathway. We have also identified a large number of hem mutants, and characterized the genes so defined. Here we concentrate on three specific systems chosen to illuminate different aspects of heme regulation. The hemA gene encodes the first enzyme specific to the heme pathway, glutamyl tRNA reductase. We are studying control of hemA gene expression during starvation for heme and in different growth conditions. We've also discovered a pair of genes, hemF and hemN, that encode alternative aerobic and anaerobic enzymes for a late step in heme synthesis, and we'll study the influence of oxygen on these genes. Finally, we've learned that S. typhimurium has two routes of ALA synthesis, and we suspect that this duality is important in regulating flow through the pathway to different end products. This study is also relevant to the larger problem of S. typhimurium's life in the colon. Most of what we know about enteric bacteria has been observed during growth in air. As a facultative anaerobe, S. typhimurium respires to electron acceptors when they are present, but can also grow by fermentation in their absence. In nature, electron acceptors may be present only intermittently (in the diet, or as oxygen diffusing from the epithelial surface). The ability for rapid development of respiratory capacity may be very important. We'll attempt to exploit heme regulation as an approach to more general questions about control of gene function by oxygen.